There are many types of catalysts, which can be classified into liquid catalysts and solid catalysts according to the state; homogeneous catalysts and multiphase catalysts according to the phase state of the reaction system; homogeneous catalysts include acid, base, soluble transition metal compounds and peroxide catalysts.

Chemical reactions that take place in the presence of a catalyst are called catalytic reactions. In a chemical reaction, some of the original chemical bonds of the reacting molecules must dissociate and form new ones, which requires a certain activation energy. In some systems where chemical reactions are difficult to occur, the addition of a third substance (catalyst) that helps rearrange the chemical bonds of the reacting molecules acts to reduce the activation energy of the reaction, thus accelerating the chemical reaction and controlling the selectivity and steric regularity of the products.

At present, the role of the catalyst is not fully understood. In most cases, it is believed that the catalyst itself participates in the chemical reaction along with the reactants, reducing the activation energy required for the reaction. Some catalytic reactions are due to the formation of easily decomposable "intermediates", when the decomposition of the catalyst restores the original chemical composition, the original reactants become products.

Definition: According to the definition proposed by IUPAC in 1981, a catalyst is a substance that accelerates the rate of a reaction without changing the standard Gibbs free enthalpy change of that reaction. This effect is called catalysis. A screening involving a catalyst is a catalytic reaction.

From the multi-component system, some of the substances are extracted with appropriate liquid agents (or water) to make catalysts with porous structures. For example, the skeleton nickel catalyst is made by melting a quantity of nickel and aluminum in an electric furnace, and the melt becomes an alloy after cooling.

Each method for making catalysts is actually a combination of a series of operating units. For convenience, the name of the key and distinctive unit of operation is used as the name of the manufacturing method. The traditional methods include mechanical mixing, precipitation, impregnation, solution evaporation, thermal melting, immersion, ion exchange, etc. The new methods developed now include chemical bonding, fibrillation, etc.